1. Field of the Invention
The present invention relates to the field of linear regulators.
2. Prior Art
Linear voltage regulators are well known in the prior art, being commonly used to receive an unregulated input voltage and to provide a regulated output voltage somewhat lower than the input voltage. Such regulators comprise a pass transistor coupled between the input to the regulator and the output of the regulator, and a pass transistor control circuit controlling the control terminal of the pass transistor based upon the comparison of a reference voltage with the output voltage of the regulator, typically as divided down by a resistor divider.
The foregoing types of linear regulators work well and are widely used. Such regulators are widely commercially available in integrated circuit form, the lower power regulators including the pass transistor as part of the integrated circuit and the higher power regulators using an external discrete pass transistor. However, unless some provision is made for the soft start of such regulators, the pass transistor will be turned on hard when power is first applied to the regulator, drawing an initial high current spike from the power supply. In that regard, substantial energy may be initially required at the output of the regulator if there is a substantial capacitive load thereon, whether because of the circuitry being driven by the regulator, or merely the presence of the typical smoothing capacitor normally provided on the output of the regulator. Substantial energy also may be initially required at the output of the regulator due to nonlinear loads where the nonlinearity is a function of voltage.
To limit the inrush current on turn-on, a resistor is commonly coupled in series with the regulator circuit, with the pass transistor control circuit sensing the voltage drop across the resistor and controlling the control terminal of the pass transistor to limit that current to a predetermined maximum value. That maximum current, of course, must be higher than the maximum expected load on the regulator in normal operation. Accordingly, when a system using such regulators is first turned on, there will be a momentary load on the power supply exceeding the largest load expected during normal operation of the system, caused by the simultaneous extraordinary inrush currents of all the circuits in the system. Further, the system itself may have various circuits, not all of which could operate at their maximum power requirements at the same time. Accordingly, the maximum normal operating power requirements from the power supply may be much less than the momentary power requirement on first turn-on of the system. Consequently, the inrush current requirements of a system can often determine the minimum power supply size, weight and cost, even though the normal operation of the system would only require a smaller, lighter and less costly supply.
In addition, in many systems it is desired to be able to replace a printed circuit board without shutting off power to the system, typically referred to as "hot swapping." In computer systems, hot swapping will allow the replacement of a board or the addition of a new board without loss of information in volatile memory, without requiring rebooting the system, etc. In systems such as communication systems and the like, wherein a plurality of boards of similar function are plugged into a motherboard, boards may be hot swapped or additional boards added without shutting down the system. This allows maintenance and upgrading without interfering with communications or other functions in channels serviced by the remaining boards in the system. In hot swapping applications, however, unless inrush currents are adequately limited, the addition of a board to a system in operation can cause a momentary power glitch which may disturb other circuits in the system.
A similar effect is encountered when a circuit is shut down. In this case, when an existing electrical load of the circuit is suddenly removed, an over-voltage condition may be imposed on the other circuits in the system, causing a temporary or permanent disruption in their operation. Also, if the load on a switching power supply if drastically reduced, the switching power supply may latch in a shutdown condition.